The present invention relates to an electrode for use with an electrophotographic photoconductor, and an electrophotographic photoconductor using the electrode.
In electrophotography, there is employed a photoconductor comprising a substrate made of, for example, glass or a plastic, an electrode in the form of an electroconductive layer formed thereon, for example, by depositing a metal or by coating an electroconductive paint, and a photoconductive layer formed on the electroconductive layer.
The material and shape of the above electrode are appropriately chosen in accordance with the characteristics of the photoconductor and the employed method of fabricating the photoconductor.
When a photoconductive layer comprising a selenium-based material is employed, in most cases aluminum or an aluminum alloy is used as the material for the electrode, which is worked into the form of a drum.
When a photoconductive layer is formed by coating a photoconductive layer coating liquid, a metal layer, deposited by vacuum evaporation or sputtering on a plastic film, may be used as the electrode. In particular, an aluminum-metallized layer formed on a polyethylene terephthalate film is widely used as an electrode of an organic photoconductor.
The reasons why aluminum is widely used as the electroconductive material for the electrode are that (1) aluminum can be relatively easily worked into a thin film on a plastic film, (2) non-ohmic contact is easily attained at the interface between an aluminum electrode and a photoconductive layer formed thereon, (3) when aluminum is employed as the material for the electrode, high charge acceptance potential can be obtained in the electrophotographic photoconductor without impairing the fundamental electrophotographic properties thereof, and (4) aluminum is not high priced.
One of the representative organic electrophotographic photoconductors (hereinafter referred to as the OPC) employed at present is of the so-called function-separation type, which comprises a substrate, an electrode formed on the substrate, a charge generating layer formed on the electrode, and a charge transporting layer formed on the charge generating layer. Specifically, a representative example of the OPC, which is most widely employed at present, comprises a substrate made of polyethylene terephthatalate, an aluminum layer serving as the electrode, a charge generating layer and a charge transporting layer which are successively overlaid on the substrate. The charge transporting layer generally comprises a triphenylamine type or hydrazone type positive-hole-moving material dissolved in a polymer. Since an electron-moving organic compound for use in the charge transporting layer that can be employed in practice has not been discovered, an electrophotographic photoconductor of the function-separation type using the OPC is usually employed under application of negative charge. In other words, in the course of the formation of a latent electrostatic image on the photoconductive layer, positive holes move from the charge generating layer toward the charge transporting layer, so that they are quenched at the surface of the photoconductive layer.
The inventors of the present invention have discovered that several metals, in particular, aluminum, employed as the electrode of an electrophotographic photoconductor under application of negative charge have the following serious shortcoming. When the surface of the photoconductive layer is charged to a negative polarity, a positive charge is induced on the back side thereof on the side of the electrode. When the photoconductive layer is exposed to a light image and the corresponding latent electrostatic image is formed thereon, the electric charges at the surface of the photoconductive layer dissipate through the electrode which is positioned adjacent to the charge generating layer. When this is repeated many times while in use, the electrode is gradually subjected to anodic oxidation. Eventually, the electrode is oxided so that the resistivity thereof highly increases, losing the function as the electrode.
In particular, when the photoconductor is in the form of a sheet comprising a transparent substrate, and charge quenching for image transfer and cleaning is performed by exposing the photoconductive layer to light from the side of the substrate, the electrode is designed so as to be transparent with a thickness of several hundred Angstroms for easy charge quenching. When the thickness of the electrode is in the above order, the electrode is almost entirely oxidized very quickly during the dissipation of electric charge from the charge generating layer into the electrode. For instance, in the case where the average spectral transmittance of an aluminum electrode in the form of a thin layer is 40% in the visible light region, the aluminum electrode is almost entirely oxidized when an electric charge of 3.times.10.sup.-2 C/cm.sup.2 has passed through the electrode.
When the electrode has a thickness greater than the above-mentioned thickness, for instance, when the thickness is in the order of micrometer, the electrode is scarcely affected by the above-mentioned oxidation. This is because the above-mentioned oxidation proceeds only at the interface between the electrode and the charge generating layer and the oxidation does not proceed to the extent that the electrode is entirely oxidized. The result is that the necessary electric conductivity of the electrode is maintained by the non-oxidized portion of the electrode. In this case, however, the electrode is not transparent at all because of the above-mentioned thickness.
Even when the photoconductive layer is positively charged for the formation of latent electrostatic image, the photoconductive layer is charged negatively for quenching the positive charge in order to facilitate image transfer to a transfer sheet or to clean the surface of the photoconductive layer. The above problem is unavoidable in both negative charging and positive charging.
Noble metals such as Au, Pt and Pd are of course resistant to oxidation. Metals such as Cr, Ni, Ti, Co and W are hardly oxidized, and even if they are oxidized, the effect of the oxidization on the electric conductivity is negligible. However, when these metals are employed as the material of the electrode, positive hole injection from the substrate to the photoconductive layer is so considerable that the charge acceptance potential of the photoconductive layer is significantly decreased and the rising of the charging of the photoconductive layer is caused to slow down, which occur before the deterioration of the photoconductive layer itself, which may be caused by the above-mentioned oxidation of the electrode.
It has been confirmed by the inventors of the present invention that Ni-based, Co-based and Fe-based alloys are resistant to acids, heat and corrosion and are relatively good materials for the electrode of electrophotographic photoconductor. In particular, Hastelloy, Monel, Illium, and Monel Metal, which are Ni-based alloys, are good. However, when these alloys are employed, more hole injection takes place as compared with the case where aluminum is employed. Further, these alloys are expensive.